1. Field of the Invention
The present invention relates to novel polytetrafluoroethylene (PTFE) fibers and a method for manufacturing the same, and more particularly relates to PTFE fibers with a reduced density.
2. Related Background Art
Since PTFE resins have a considerably high melting viscosity and are not dissolved by most solvents, fibers cannot be produced by a generally adopted method such as extrusion spinning of molten resins and resin solutions. Therefore, various specific manufacturing methods have been adopted conventionally. U.S. Pat. No. 2,772,444 proposes a method for manufacturing a PTFE fiber by emulsion spinning of a mixed solution of an aqueous dispersion solution of PTFE fine particles and viscose, followed by sintering of the PTFE at high temperatures, while removing the viscose by thermal decomposition. However, the manufacturing cost of the PTFE by this method is high, whereas the strength of the fiber obtained is low, and therefore the strength of a product obtained by processing this fiber as a raw material also is low.
U.S. Pat. No. 3,953,566 and U.S. Pat. No. 4,187,390, for example, propose a method for manufacturing a high-strength PTFE fiber by slitting a PTFE film or sheet into a minute width, followed by stretching of the obtained tape. However, this method has a difficulty in maintaining a width of the tape obtained by slitting uniformly along the lengthwise direction. Also, there exists a problem that an end portion of the tape tends to be a fibril. For these reasons, there exists another problem that the fiber may break partially during the step of stretching the tape with a high degree.
U.S. Pat. No. 5,562,986 proposes a method for manufacturing cotton-like materials made of PTFE fibers having a branch structure by opening a uniaxially stretched article, specifically a uniaxially stretched film of a molded PTFE article by a mechanical force using a pin roll with a needle density of 20 to 100 needles/cm2. According to this method, however, a length of the obtained PTFE fibers mostly is not more than 150 mm, and it is difficult to obtain a PTFE filament.
WO96-00807 proposes a method for manufacturing cotton-like materials made of PTFE fibers having a branch structure by opening a uniaxially stretched film of a molded PTFE article by a mechanical force. According to this method, however, a density of the obtained PTFE fibers becomes a high specific gravity exceeding 2.15 g/cm3, thus making it difficult to obtain a light-weight final product.